Updated 24 Jun 2011

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Introduction

Improve your living environment by drying, warming and filtering indoor air with a dehumidifier.

We've tested 9 models of various sizes. 8 models use conventional ("refrigeration") technology and 1 uses the new “desiccant” technology.

Our testing found a particularly effective dehumidifier – but the downside is it uses more energy.

Models we tested

This report contains test results and recommendations for the following dehumidifiers:

Controlling condensation

If you've got a problem with condensation, the first thing you need to do is consider where the water is coming from. Here are some of the common problem spots.

Sources of moisture

Activity	Litres
Cooking	3.0 per day
Clothes washing	0.5 per day
Showers and baths	1.5 per day (per person)
Dishes	1.0 per day
Clothes drying (unvented)	5.0 per load
Gas heater (unflued)	up to 1.0 per hour
Breathing, Active	0.2 per hour (per person)
Breathing, Asleep	0.02 per hour (per person)
Perspiration	0.03 per hour
Pot plants	as much as you give them

Want to check the humidity in your house?
Inexpensive ($12 plus freight) humidity gauges are available that let you check humidity levels in your house – useful for finding the sources of dampness. See www.thermometer.co.nz.

Improving condensation problems

• Keep rooms ventilated and warm during winter - at least 7°C warmer than outside temperatures. Install heavy curtains and draw them at night: it helps keep the home warm and reduces the number of cool surfaces for water vapour to condense on. Leave windows closed on damp days.
• Dehumidifiers have often been teamed with unflued LPG heaters. One spews out moisture (and other contaminants) while the second mops up the moisture. No, no, no. It’s not the dehumidifier that’s the problem here – it’s the unflued LPG heater. Their exposed flame is a fire risk and they fill the house with water and other harmful contaminants. Don’t use them. Use cheaper to run and safer electric heaters.
• Always use close fitting lids on pots when cooking and fit extractor fans over the cooktop or stove, and in the bathroom (these must be ducted to the outside).
• Use a cupboard heater in problem wardrobes. A ventilation grille in the top of the wardrobe will also help.
• Vent clothes dryers to the outside and don't hang clothes inside to dry. Close doors when cooking, showering or using the dryer, to limit the spread of moist air.
• Limit the number and size of pot plants in the house, .
• Check the ground under your house is dry. If it's wet, cover with polythene (if feasible), taping the joints, and ensuring a tight fit around piles. Check that drainage systems are diverting water away.
• Fix any leaks in the roof or around windows.

How they work

Most dehumidifiers use a small refrigeration unit to first cool the room’s air (so it can condense the moisture) then reheat it. The water-removal rate reduces as the air temperature drops.

Desiccant dehumidifiers use a water-absorbing (desiccant) material such as a silica-gel. The air in the room is blown through a slowly rotating disc containing the desiccant – that’s what dries the air.

Next, the moisture-laden desiccant disc rotates into a chamber where a heated portion of the desiccant-dried air is blown through the disc. This dries the desiccant. The desiccant then rotates out of the drying chamber and repeats the cycle.

After this, the damp and warm air that’s resulted from the disc-drying process passes through a heat exchanger and is cooled by air coming in from the room. The result is that moisture in the previously warm air is condensed out and collected in a tank.

It sounds complicated, but it works – especially at low temperatures.

Warm and dry?

All the electricity a dehumidifier uses ends up heating the room. This is a good thing in our climate … but it gets even better.

Every time water is turned from a liquid to a gas, heat has to be added. This heat is called the “latent heat of evaporation”. The reverse happens when water vapour is condensed to a liquid – that latent heat is released.

So when a dehumidifier condenses the water vapour in the air back to a liquid for draining off, the latent heat in the water vapour is released. The dehumidifier both dries the air and delivers more heat to the room than it uses in electricity. Only a heat pump gives you more bang for your buck.

Checklist

Before buying a dehumidifier you should do everything possible to reduce the causes of dampness. Underfloor water barriers, improved insulation as well as kitchen and bathroom ventilation are all good stuff. But for most homeowners these are longer-term projects – and most are impossible if you are renting.

A dehumidifier starts improving your home environment the moment you switch it on. And you can take it with you if you move house.

Features

If you're thinking of buying a dehumidifier, consider these features:

Low temperature effectiveness
You can get excessive moisture in the air at high and low temperatures, and will want a machine that can deal with both. However, makers usually give water removal rates at conditions which make their machines look the best, typically 30°C and 80 percent relative humidity (RH).

For example, a machine may claim to remove 16 litres a day at 30°C and 80 percent RH. The figures sound impressive, but are misleading for anyone wanting to deal with winter condensation, when temperatures get below 15°C. Try asking for figures at 10°C and 15°C, although they will not always be available.

Using a heater and a dehumidifier together will heat a cold and damp room more quickly and cheaply than using just a heater.

Mobility
Look for a model with good castors and a handle or hand-grips to ensure easy movement. Models can range in weight from 10 to 25 kilograms. If you have stairs, shop around for a model that's not too heavy for you. If space is a concern, check the dimensions. Some take up much less space than others.

Tank
There is usually a trade-off between mobility and size. Small water-collection tanks make a dehumidifier more compact, lighter and easier to carry around. But if the tank is too small, you'll have to empty it several times a day.

Models with larger tanks won't need to be emptied as often but can take up more floor area, and a big tank full of water can be difficult to manoeuvre to the emptying point.

Models with a front tank are easier to empty. Check that it is easy to remove, and big enough to hold at least five litres of water. Some models have a drain hose adaptor, which removes the water as it's collected, so it can be poured straight down a drain.

Controls and labels
Make sure they are easy to see, understand and use. An adjustable humidistat allows you to set humidity levels.

Rotary dial humidistats with no precise markings make it hard to find the actual humidity level. If this is particularly important (as it may be for some people with allergies, for example) you can buy a humidity gauge to check humidity levels. See www.thermometer.co.nz.

Noise
Most dehumidifiers make upward of 50 decibels (dBA) of noise - enough to be annoying if you're trying to watch TV, hold a conversation or sleep. (A reasonably quiet fridge makes about 39-40dBA.)